If I were going into battle and no considerations were taken in account(such as availability) other then sheer performance and weight it would be the 6.5 grendel. It can do anything the .308 can do and you can carry more of it.
The .50 BMG is one heck of a round and the Barrett company makes some fine rifles. So many guns...so little money.
I assume your talking about grains of water....a common method for determining internal space. the reason for the greater space is the blown out shoulder and neck to make the .50 a straight wall case...a .50 caliber heavy bullet needs greater powder space to just maintain velocity. as much as a .50 caliber is a thumper it is not a long range weapon. 2 of my favorite rifles are chambered for the .45-70 government round. My trap door springfield is loaded up to about half the velocity of my lever action marlin...the chambering in its original 19th century form is grossly underpowered considering the case space and of course if I were to accidental switch rounds I would blow my trapdoor and myself to kingdom come....
......the .50 beowolf is maximized and cannot go any faster than it goes now. with such a poor aerodynamic design and relatively slow velocity this round is one great close range thumper but not something to use if your fighting the kinds of battles you fight in Iraq where the ranges go out to the thousands of yards....it would have made an interesting round in Vietnam.
not an AR fan....I really dont know the difference between a 6.5 grendel and a 6.8 SPC Remington but both look head and shoulders above the 5.56. but except for carry ability I see the the .308 superior round and subscribe to Col. Cooper's aimed fire paradigm versus the "Spray and Pray" method made popular in vietnam. take aimed shots and you dont need so much ammo to carry. But I am set in my ways and will defer to the services for whatever they choose as long as they keep winning battles...just keep politicians out of the mix, like the whole M-16 debacle where the politicians made the determination what the grunts on the ground needed to carry.